With a development of technologies, more mobile terminals (e.g., a mobile phone) have a music playing function besides a traditional audio playing apparatus. When playing music, different equalizer modes (e.g. normal, classical, pop, country, rock, etc.) are achieved by performing an amplitude amplification or attenuation on the original sound signal in certain frequency bands. Usually, configuration schemes of the different equalizer modes are obtained by measuring a frequency response curve.
When listening to music under a condition that the external environment is noisy, a user generally increases the volume to clearly hearing the music. Usually, the volume can be increased by increasing output magnification of a power amplifier. However, whether it is a speaker or an earphone, a voltage which exceeds a rated power may cause a burn out of a sound device due to overheating. Also, hearing an excessive volume for an extended time will gradually damage a user's hearing. In order to protect the user's hearing, and prevent damage of the sound device, it may be necessary to manage RMS voltage or sound pressure level when the sound device operates under the maximum volume, so that the RMS voltage or sound pressure level is lower than a specific limit (hereinafter referred to as a protection limit).
Commonly, in a case where the equalizer mode is turned off, the frequency response curve in the working band is planar. However, for other modes, the frequency response curve may have a variance of −6 dB to +6 dB relative to equalize OFF or NORMAL mode. There are several methods for avoiding the output exceeding the protection limit. One method sets a gain in the maximum volume in accordance with the situation in which the equalizer mode is turned off (i.e., an equalizer curve is planar), so that an output is lower than the protection limit. However, since equalizer curves of some equalizer modes are higher than the planar equalizer curve, it is possible that the output voltage still exceeds the protection limit. A second method whose set manner is the same as that of the first method but lowers the protection limit by −6 dB ensures the output is lower than the protection limit. As such, the volume in any equalizer mode will be lower than the volume in which the equalizer mode is turned off, thus the same SNR (Signal to Noise Ratio) cannot be achieved under a noisy environment, and the equal quality of sound cannot be obtained. A third method whose set manner is same as that of the first method in the situation in which the equalizer mode is turned off, while lowering the protection limit by −6 dB when the equalizer mode is turned on. The method ensures the output is lower than the protection limit, while it may cause a jump of the volume when switching the equalizer mode on or off.
In addition, the three methods described above cannot ensure that the output is lower than the protection limit when a sound field mode (such as an echo enhancement, a 3D sound effect) is used. FIGS. 1A to 1C illustrate the frequency response curves in the above three methods.